In an apparatus used conventionally, as a magneto-optical anistropy detector of a magnetic medium, for reading out information recorded in accordance with the magnetized state of the magnetic medium, when the linearly polarized light passes through the magnetic medium or is reflected on the surface of the magnetic medium, the incident linearly polarized light is slightly converted into the elliptically polarized light and the principal axis of the ellipse slightly rotates from the polarizing direction of the incident linearly polarized light. Conventionally, the rotational angle of the principal axis of polarization is measured using a polarizing element and a polarization analyzer, but this method involves the drawbacks in that the apparatus is complicated and a signal-to-noise ratio is low.